Thyrotropin-releasing hormone (TRH) from hypothalamus is the major regulator of TSH synthesis and secretion. Most recently, TRH and ppTRH mRNA have been identified in rat testis. However, at present none of the potential extra-pituitary actions of TRH in a variety of structures including testis has been elucidated. The central hypothesis of this proposal is that testicular TRH and/or its related peptides are important autocrine and/or paracrine regulators of testicular development, steroidogenesis, and spermatogenesis. To test this hypothesis, rat testicular TRH and TRH related peptides including TRH25-50, TRH83-106, and TRH160-169 will be characterized, and the levels of testicular TRH and its related peptides through different developmental stages of the testis will be examined. The specific cellular locations of TRH and TRH related peptides, and ppTRH mRNA in the testis will be determined by immunohistochemistry and in situ hybridization, respectively. To evaluate the role of TRH in the testis, testicular TRH receptor and TRH receptor gene expression will be characterized and localized to specific cell types by autoradiography and in situ hybridization. Gonadotropin regulation of TRH and TRH gene expression, as well as TRH receptor binding activity and the gene expression in rat testis, will be investigated by in vivo and in vitro studies. The biological role(s) of TRH and TRH related peptides in the regulation of testicular functions will be investigated by in vitro studies with specific attention to the regulation of testis development, testosterone production, and spermatogenesis. The studies herein will provide new insight concerning the potential biological functions of TRH and/or its related peptides in the testis. New knowledge will be gained about the cellular location and hormonal regulation of TRH and its receptors as well as the TRH gene and TRH receptor gene expression. TRH, which appears to play an inhibitory role in the regulation of testosterone production, and is a strong candidate for the regulation of spermatogenesis in the rat, has now been demonstrated in the human testis. Thus, these studies have implications for man as well, since TRH may play a role not only in normal intra-testicular regulation, but also in pathological states.